Various kinds of rustproof materials are widely used in various fields in order to reduce an environmental load, cut down life-cycle cost, and further ensure safety. Their use examples include small components such as electronic components, home electric appliances, automobiles, construction materials, and further large structures such as infrastructure facilities.
A plated steel material is one of the rustproof materials, and especially a Zn-plated steel material is often used. Main reasons why the Zn-plated steel material is often used are that it is relatively low-priced and has a sacrificial anticorrosive action on a base iron, and that a corrosion rate of Zn itself in an atmospheric environment is low. Examples of a plated steel material other than the Zn-plated steel material include an Al-plated steel material. However, the Al-plated steel material has problems that it does not have a sacrificial anticorrosive action on a base iron due to an oxide coating film existing on a surface of its Al plating layer, and that it is poor in post-coating corrosion resistance since its chemical conversion treatability is insufficient due to the oxide coating film existing on the surface of the Al plating layer. On the other hand, since Al itself is lower in corrosion rate than Zn, it is thought that the application range widens if sacrificial corrosion resistance and post-coating corrosion resistance can be ensured in the Al plating layer.
Therefore, arts for improving corrosion resistance of an Al plating layer have been proposed. For example, Japanese Laid-open Patent Publication No. 2003-34845 (Patent Literature 1) discusses that, by making 0.5 to 10% Mg contained in Al plating, a sufficient amount of a chemically converted coating film is generated, so that post-coating corrosion resistance improves. However, adding an active element such as Mg to the Al plating increases activity of the Al plating layer itself, so that bare corrosion resistance (corrosion resistance in an uncoated state) deteriorates on the contrary.
Further, Japanese Laid-open Patent Publication No. 2007-302982 (Patent Literature 2) describes that applying Zn, a compound of Zn, or the like on a surface of an Al plating layer improves post-coating corrosion resistance. However, an effect of improving post-coating corrosion resistance is still insufficient since adhesion of Zn or the Zn compound to plating is not sufficient.